The Hurst Boiler Series 100 3-Pass Wet Back Firebox Packaged Boiler is a three-pass, fully packaged firebox boiler built out of the thickest boiler steel in the industry. Its fuel efficiency and flexibility also match that of Series 45, and low-NOx configurations are available. The 100, however, can function as a steam or water boiler and comes with a deluxe “trim” for each function. Steam/water trim each offers low water cut-off, ASME safety valve, operating/upper limit pressure controls, and other helpful gauges/management tools.